Brief Description of the Prior Art
The use of light or optical conductors in place of, or in addition to, electrical conductors for the purpose of long distance transmission of information signals has rapidly increased in recent times. Glass fibers or filaments are preferably used as such light conductors, as are some other solid and liquid substances. Very long fiber-like conductors are frequently referred to as optical fiber cables, or as light conducting cables.
Optical fiber usage presents some problems due to the mechanical characteristics of these fibers, such as their low tensile strength and ultimate enlongation. One effort made to reduce or solve these problems entails the encasement of the optical fibers in tubular elements or sheaths of various types (often termed innerducts) which both protects the optical fibers from mechanical damage, and also facilitates their placement in underground systems extending over great distances. Among the sheathing materials which have been employed are thermoplastic synthetic resins such as polyethelene, polyester, polyamides and polyacrylates. It is still necessary, however, to attempt to minimize the magnitude of the stresses and deforming forces which act upon the optical fibers during the construction of cables, including the tubular protective elements, and during the laying of these cables in the earth.
In U.S. Pat. No. 4,153,332 to Longoni, a sheathed optical fiber cable is disclosed and includes a surrounding external sheath of a plastic material which encloses a plurality of tubular elements. Each one of the tubular elements encloses an optical fiber. The tubular elements have smooth walls, both internally and externally. The Longoni patent expalins that the optical fibers which are inserted in the tubular elements or sheaths can be bare, or each can be covered with a thin adhering protective layer of a plastic material.
Franke et al U.S. Pat. No. 4,039,248 describes a fiber-like conductor which is housed within a tubular sheath. The sheath may be corrugated for the purpose of retaining the fiber-like conductor in a desired undulating configuration within the sheath.